mdx: add remark-frontmatter to pipeline, fix rehype-pretty-code line numbers visitor

lib/mdx-hast-visitor.js

@@ -0,0 +1,26 @@
+/**
+ * HAST visitor plugin that adds data-line-numbers attribute to code blocks
+ * inside <pre> elements.
+ */
+import { visit } from 'unist-util-visit'
+
+/**
+ * Rehype plugin that adds line number indicators to code blocks.
+ * @returns {import('unified').Plugin<[], import('hast').Root>}
+ */
+export default function addLineNumbers() {
+  return function attacher(tree) {
+    visit(tree, 'element', function visitor(node) {
+      if (
+        node.tagName === 'pre' &&
+        node.children?.length === 1 &&
+        node.children[0].type === 'element' &&
+        node.children[0].tagName === 'code'
+      ) {
+        const code = node.children[0]
+        code.properties = code.properties || {}
+        code.properties['data-line-numbers'] = ''
+      }
+    })
+  }
+}